Hikurangi tsunami warning at a coastal school
At 8:12 am, a M7.2 earthquake ruptures the Hikurangi subduction interface 60 km offshore and 25 km deep. The shaking at a coastal school lasts about 70 seconds and is strong enough to make some students unsteady. The school is 500 m from the beach and 8 m above sea level; the field trip group is already on the sand. A national tsunami warning arrives by phone at 8:20 am, saying 'first wave expected at 8:57 am, heights up to 3 m above normal tide, evacuate all coastal areas.' The school has a tsunami plan: assemble on the upper field, then walk 1.2 km inland to a designated safe zone at 35 m elevation. Some staff argue that because the first wave is 45 minutes away, there is time to board buses. Others say the road along the coast is congested. The school roll is 640, and 30 students are on the beach with two teachers. It is a calm, sunny day; the tide is high at 9:30 am.
1. What should the school do immediately, and why? Include timing and route.
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Move the beach group inland and uphill immediately, not to the school field. The 70 seconds of strong shaking is a natural warning; local waves can arrive before official warnings, and the 8:57 estimate is only a forecast. The upper field at 8 m is within the likely run-up zone for a 3 m wave, especially with high tide at 9:30 and possible amplification. The designated safe zone at 35 m, 1.2 km inland, is the correct destination. Because the beach group is closest, they should move first, on foot, away from the coast and not wait for buses; buses on the coastal road could be trapped by congestion or the first wave. The rest of the school should evacuate vertically if there is not enough time to reach 35 m, otherwise walk inland. Take the roll at the safe zone and do not return until an official all-clear.
2. A staff member says 'the warning says 3 m, and we are 8 m up, so the field is safe.' Evaluate this reasoning.
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The reasoning confuses wave height with run-up and elevation. A 3 m wave at sea can run up higher than 8 m depending on beach slope, bathymetry, tide and debris; the school is only 500 m from the beach, so it is in the evacuation zone. The 8 m elevation is not a vertical wall, and the field may be lower than the quoted height or connected to the beach by a low route. Tsunami height forecasts have uncertainty; the first wave may not be the largest. The safe zone at 35 m is chosen for a reason. The correct judgement is to treat 8 m as insufficient, not to use one number to overrule the official evacuation zone.